1. Field of the Invention
This invention relates to an oil well pipe threaded joint as a connection portion for tubes used for the excavation of a crude oil or a natural gas. More particularly, it relates to an oil well pipe threaded joint which can prevent the occurrence of looping or squeezing of a ring (also called a "ring-like buffer") interposed between pins of oil well pipes for improving the sealability of the oil well pipe threaded joint and damage such as corrosion due to a corrosive fluid inside a pipe.
2. Description of the Prior Art
To excavate crude oil or natural gas, it has been customary to drill an oil well (also called an "ore chute") from the ground surface to an oil layer or a gas layer, to apply a casing round the inner peripheral surface of this oil well so as to prevent the collapse of the oil well and to recover the crude oil or the natural gas through tubing inserted into the casing. Because the depth of the oil well is generally as great as several thousands of meters, steel pipes of a low alloy steel, etc, are fastened to one another by various methods to obtain an elongated pipe. Here, the connection portion of the tubing, etc, is referred to as an "oil well pipe threaded joint".
Various oil well pipe threaded joints are known as described below. 1 Japanese Unexamined Utility Model Publication (Kokai) No. 58-142475 proposes an oil well pipe threaded joint wherein a pin having an outside thread is formed on an outer peripheral surface in the proximity of an end portion of each pipe and a coupling (also called a "sleeve") having an inside thread on an inner peripheral surface in the proximity of an open portion of a cylindrical member is formed so that the pin of each pipe can be threaded to this coupling. In this case, since the fitting and removal works are repeatedly carried out between each pipe and the coupling, a coating layer is likely to be damaged, and a ring made of a soft thermoplastic resin such as Teflon (trade name) is used. However, there remains the problem that the thermoplastic resin ring cannot play the role of a so-called "torque shoulder" for absorbing a torque acting on the distal end surface of each pipe and between both side surfaces of a protuberance portion at the time of makeup (also called "fastening") and for reliably securing fixing. 2 Japanese Examined Utility Model Publication (Kokoku) No. 6-49989 proposes a threaded joint wherein a pin having an outside thread formed on the outer peripheral surface of one of the pipes in the proximity of the end portion thereof and a pin having an inside thread formed on the inner peripheral surface of the other pipe in the proximity of the end portion thereof are interengaged with each other.
Recently, a so-called "water injection well" which injects water or brine (hereinafter called "injection water") into the oil layer through the tubing has been widely employed in order to improve a crude oil recovery ratio. When ordinary water or brine is injected into the tubing, however, the tubing (or the oil well pipe) is remarkably corroded due to a high oxygen content of this injection water, and the collapse of the tubing occurs within a relatively short period. Therefore, to economically prevent the collapse of the tubing, 3 Japanese Unexamined Patent Publication (Kokai) No. 62-98088, Japanese Unexamined Utility Model Publication (Kokai) No. 4-63884, U.S. Pat. No. 5,236,230, etc, propose a so-called "inner surface coating tubing" which applies a corrosion-resistant material such as a synthetic resin or ceramic to the inner peripheral surface of the oil well pipe made of ordinary steel, etc, as a coating, and protects the tubing from a corrosive fluid inside the tubing such as crude oil.
Nonetheless, the conventional oil well pipe threaded joints described above are not free from detrimental deformation under the environment of use such as looping (also called "buckling" or "jump-out") and squeezing (also called "swell-out").
Here, the mechanism of looping is as follows. When the corrosive fluid (also called the "pressure medium") such as the crude oil or the natural gas inside the tubing enters the gap between a recess 55 defined by the distal end surface 51h, 52h of a pin 51a, 52a of each pipe 51, 52 and a groove 53f of a coupling 53 and a ring 54 fitted into this recess 55 and as a result, when the pressure of the pressure medium inside the tubing (also called the "pipe internal pressure) drops with respect to the pressure of the pressure medium (hereinafter called the "back pressure") as shown in FIG. 1(a), the back pressure is applied to the end surface of the ring 54 on the outside diameter side as indicated by and arrow in FIG. 1(a), so that the ring 54 is pressurized in a direction indicated by an arrow F' in FIG. 1(a) and partially jumps into the tubing as shown at the portion F in FIG. 1(b).
The mechanism of squeezing is as follows. During the makeup (or "fastening") work of the oil well pipe threaded joint, the distal end surfaces 51h and 52h of the pins 51a and 52a of the pipes 51 and 52 cause the ring 54 to interfere in the same direction as the axial direction of the tubing (hereinafter called the "axial direction"), or the groove 53f of the coupling 53 causes the ring 54 to interfere in the inner direction of the pipe as shown in FIG. 2. As a result, the ring 54 undergoes elastic deformation from the initial shape represented by dash line in FIG. 2 to the shape represented by solid line in FIG. 2 and swells out at the portion G in FIG. 2 in the direction represented by arrow G'.
Once such looping and squeezing occur, a tool lowered inside the tubing during a logging (or "drift") operation is caught by the ring which jumps out or swells out inside the tubing, and the logging operation cannot be carried out smoothly and the operation factor is remarkably lowered. When looping and squeezing occur in the oil well pipe threaded joint, the surface of the pin and the coupling which is originally covered with the ring is exposed, and this exposed surface comes into contact with the corrosive fluid and is eroded. As a result, the collapse of the tubing occurs within a short time, and reliability drastically drops.
To solve the problems described above, 4 a method which uses a so-called "retainer ring" by making a spring or a metallic ring composite with a synthetic resin has been proposed. 5 On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 59-126191 proposes an oil well pipe joint having the construction wherein a lip portion having a tapered outer peripheral seal surface and a distal end seal surface is formed at an end portion of a pipe having an external thread, a shoulder portion having an inner peripheral seal surface and a butt seal surface corresponding to the outer peripheral seal surface and the distal end seal surface, respectively, is formed at the inner depth of a coupling having an internal thread engaging with the external thread, and stopper portions are formed in the proximity of the outer and inner peripheral seal surfaces in such a manner as to engage with each other.
6 Further, Japanese Examined Utility Model (Kokoku) No. 1-12051 proposes an oil well pipe threaded joint for engaging and connecting the pipe end portions with each other, which can butt the terminal portion of shoulder portion of the pipe end on the external thread side with that of the pipe end of the internal thread, and wherein a clearance is generated between the terminal portion of the shoulder portion of the pipe end on the internal thread side and that of the pipe end on the external thread side at the time of butting and moreover, the distal end outer surface portion at the pipe end on the external thread side and the depth inner surface of the pipe end of the internal thread side can be brought into pressure contact with each other. In this threaded joint for the oil well pipe, the threaded joint of the reference is characterized in that an inwardly inclined taper is formed on the shoulder portion at the pipe end on the internal thread side, an R-shaped recess is formed on the pipe end inner surface portion near this shoulder portion, and a packing material is interposed into the clearance between the shoulder portion and the terminal end portion of the pipe end on the external thread side so that both of them can be butted to each other.
According to the method of 4, however, the fitting work of the retainer ring is necessary. In addition, because the retainer ring has a metallic ring, it is difficult to cause deformation of the retainer ring, so that the fitting work is extremely difficult and the working factor is low. Moreover, since the retainer ring is expensive, the production cost becomes very high. In the oil well pipe joint of 5 Japanese Unexamined Patent Publication (Kokai) No. 59-126191, a corrosive fluid is likely to enter the so-called "metal seal" between the distal end seal surface of the lip portion of the pipe and the butt seal surface of the shoulder portion of the coupling because the ring is not used and as a result, the oil well pipe joint is easily corroded and the reliability is low. Further, because the distal end surface of the lip portion of the pipe is inclined at an obtuse angle with respect to the inner wall surface of the pipe, looping and squeezing described above cannot be prevented even when a ring is fitted between the distal end seal surface and the butt seal surface. Further, in the oil well pipe threaded joint of 6 Japanese Examined Utility Model Publication (Kokoku) No. 1-12051, only the shoulder portion at the pipe end on the internal thread side is inclined inward. Therefore, looping and squeezing of the ring cannot be sufficiently prevented.